In flip-chip bonding, the contact elements on the contact surfaces or the bond paths have a typical height of 15 μm to 75 μm and are produced in the form of stud bumps, solder bumps or by the “plating process.” For the ultrasonic flip-chip bonding primarily stud bumps and contact elements in accordance with the “plating process” are used. The space or gap remaining after the bonding between the substrate and the silicon for the chip amounts usually to 50% to 70% of the starting gap dimension. So that forces between the substrate and the chip, which can arise because of the different thermal expansion coefficients of the substrate and the chip (“thermal mismatch”) can be taken up (the shear stress would otherwise loosen the chip from the substrate), it is known to introduce with a very fine needle between the chip and the substrate an additional adhesive subsequent to the bonding to increase the mechanical stability of the entire assembly. This process is characterized as underfilling or underfill.
Especially with very small distances between the chip and substrate and a small space (pitch) between the individual contact elements, this underfill process can no longer be used since the remaining gap does not allow a homogeneous underfilling with adhesive.